Clinical quantitative flow cytometry is a new technology that utilizes specialized antibody staining techniques and fluorescent standards to quantitate antibody binding to different cell populations within a sample. The Flow Cytometry Unit uses this technique to quantitate tumor cell antibody binding capacity in patients undergoing various antibody based therapies. The flow cytometric assays are performed in place of previous studies using radioactively labeled antibodies. The flow cytometric assays are more rapid, make the use of radioactivity unnecessary and are performed on 100 uL of blood instead of 25mL. The data collected is also more precise and improves the ability of NCI investigators to compare antibody binding to tumor cells to treatment response. Quantitation of fluorescent antibody binding immediately post therapy also allows precise determination of saturation of antigen sites with non labeled therapeutic antibody. Antibody based therapy is now incorporated into routine treatment for patients with leukemia or lymphoma and the majority of NCI lymphoma and leukemia protocols include it in their regimens. Antigen Quantitation is therefore a tremendous resource for NCI investigators that is available at other institutions. Quantitation of antigens prior to, during and post therapy may provide insight into pathways disrupted by novel therapeutic agents. Flow cytometric testing is also utilized to evaluate patients receiving chimeric antigen receptor (CAR) T cell therapy at NCI. It plays a vital role in assessing response to chimeric antigen receptor (CAR) T cell therapy by quantitating expansion of the therapeutic CAR T cells in the patient while at the same time monitoring the decrease in neoplastic cells.